Low back pain constitutes a devastating economic burden for individuals and society. In industry, it is the most frequent cause of disability: the number of working days lost per year in the United States is in excess of 100 million. Kramer, J., Intervertebral Disk Disease. Stuttgart, George Thieme Verlag (1981). Over 500,000 workers are affected in the U.S. each year, costing over 20 billion dollars. Kranzler, L. I., L. et al., Neurologic Clinics 3: 2 (1985). Low back pain is almost always associated with pathological changes in one or more intervertebral disks of the lumbar spine.
Intervertebral disks are a specialized fibrocartilaginous connective tissue whose function is to resist the compressive, rotational and tensile stresses applied to the vertebral column. Humzah, M. D. et al., Anat. Rec. 220(4): 337–56 (1988). These disks act as a hydrostatic shock absorber that cushions the forces generated between two vertebrae and help these vertebrae articulate smoothly with one another. The disk is a complex structure consisting of two interdependent, but morphologically distinct regions: the nucleus pulposus (NP), an inner gelatinous cushion rich in proteoglycans (PGs), and an outer annulus fibrosus (AF) made up of concentric lamellae rich in collagen fibers.
The metabolism of the cells that produce and maintain the extracellular matrix of both the NP and AF is poorly understood. The matrix in the NP is very similar to that found in articular cartilage. Jahnke, M. R. et al., Biochemical Journal 251(2): 347–56 (1988). It is synthesized and maintained throughout adult life by relatively few cells. More than 75% of NP cells are chondrocyte-like, but a significant number of large notochordal cells are present, especially prior to adult life. Maldonado, B. A. et al., Journal of Orthopaedic Research 10(5): 677–90 (1992). It is not clear if both NP cell types synthesize the large-molecular-weight hydrophilic PG, termed aggrecan, that constitutes the most abundant molecule in the tissue. As in articular cartilage, these aggrecan molecules interact extracellularly with long linear stands of hyaluronan (HA), forming aggregates that become entangled in a fibrillar network made up principally of type II collagen. Thonar, E. J. et al., Rheum. Dis. Clin. North Am. 19(3): 635–57 (1993). The swelling, fluid- and ion-transport properties, and the intrinsic mechanical properties of the collagen-aggrecan solid matrix govern the deformational behavior of the NP. The collagen network gives the tissue tensile strength and hinders expansion of the viscoelastic, under-hydrated, aggrecan molecules that provide compressive stiffness and enable the tissue to undergo reversible deformation.
The AF contains a relatively homogeneous population of chondrocyte-like cells (Maldonado, B. A. et al., Journal of Orthopaedic Research 10(5): 677–90 (1992)) that synthesize a matrix richer in collagen and poorer in PGs than cells from the NP. Importantly, some of the cells synthesize PG and collagen molecules not normally found in significant amounts in cartilage. Wu, J. J. et al., Biochemical Journal 248(2): 373–81 (1987); Mayne, R. and Brewton, R. G., “Extracellular matrix of cartilage collagen,” in Joint Cartilage Degradation. Basic and Clinical Aspects (Woessner, J. R. and Howell, D. S., eds.), Marcel Dekker, Inc., New York, pp. 81–108 (1993); Thonar, E. J.-M. A. et al., “Body fluid markers of cartilage changes in osteoarthritis,” in Rheumatic Disease Clinics of North America: Osteoarthritis (Moskowitz, R., ed.), W. B. Saunders Co., Philadelphia, pp. 634–658 (1993). The AF is thus usually classified as a fibrocartilage: it is built for strength rather than to provide reversible deformation.
The metabolism of intervertebral disk cells is much less well known than that of chondrocytes from articular cartilage. Progress in this area has been limited by the costly nature of in vivo experimental approaches and the restrictions imposed in the past by the lack of an appropriate culture system to study the metabolism of the cells. Chiba et al. (Chiba, K. et al., “Nucleus pulposus and annulus fibrosus cells cultured in alginate: characterization of matrix metabolism in different compartments,” Transactions of 2nd Combined Meeting of the Orthopaedic Research Societies of U.S.A., Japan, Canada and Europe, p. 32 (1995)) have developed a cell culture system that takes advantage of an alginate bead culture system that was developed and refined to study the metabolism of phenotypically-stable chondrocytes and the turnover of the matrix they form de novo. Häuselmann, H. J., et al., Matrix 12(2): 116–29 (1992); Häuselmann, H. J. et al., J. Cell Sci. 107: 17–27 (1994); Mok, S. S. et al., J. Biol. Chem. 269(52): 33021–7(1994); Petit, B., et al., Experimental Cell Research 225: 151–161 (1996). As in articular chondrocytes, intervertebral disk cells entrapped in these alginate beads also reform an extracellular matrix. Chiba, K., et al. Spine 22(24): 2885–93 (1997). This cell culture system can be used to study the effect of compounds on the metabolism of both AF and NP cells. This cell culture system can distinguish between changes occurring in the metabolically active (cell-associated) and-inactive (further removed) compartments of the matrix. Chiba et al. have also shown that it is feasible to entrap a whole rabbit intervertebral disk in alginate gel. Chiba, K., G. B. J. Andersson, et al., Ortho. Res. Soc. Trans. 21:190 (1996). This approach, which more closely mimics the in vivo situation, leads to improved retention of the disk structure and promotes high metabolic activities.
Lumbar intervertebral disk herniation is one of the most common causes of lower back pain. Disk herniation is initially conservatively treated with physical therapy, but other more invasive treatment modalities are sometimes required. For example, chemonucleolysis, the dissolution of intervertebral disk tissue using a locally injected enzyme, has been used for over thirty years. Olmarker, et al. Clin. Orthopaedics and Related Res. 257:274 (1990). As PGs contribute most of the swelling pressure in the NP, intradiscal injections of enzymes that degrade PGs (Bradford, D. S., et al., Journal of Bone and Joint Surgery—American Volume 65(9): 1220–31 (1983); Hill, G. M. et al., Clinical Orthopaedics and Related Researches 225: 229–233 (1987)) or collagens cause a decompression of the nerve root entrapped by the herniated mass, and thus help relieve pain. There is preliminary histological evidence that the cells in disks treated with these enzymes can replenish the NP with PGs, (Bradford, D. S., et al., Journal of Bone and Joint Surgery-American Volume 65(9): 1220–31 (1983)) helping to reestablish the shock-absorbing properties of the intervertebral cushion and, most importantly, to normalize forces and stresses placed upon adjacent disks.
Complications can arise from chemonucleolysis when it is performed with two commonly used enzymes, namely chymopapain and collagenase. Kitchel and Brown report that chymopapain treatment can lead to subarachnoid hemorrhage, paraplegia, anaphylaxis, and even death. Clin. Orthopaedics and Related Res. 284:63 (1992). Olmarker et al. note that the injected chymopapain is both neurotoxic and allergenic, and that treatment with collagenase may lead to neurologic deficits. Clin. Orthopaedics and Related Res. 257:274 (1990).
Because the painful sciatica associated with disk herniation is often not relieved by conservative treatments, including physical therapy and chemonucleolysis, many patients undergo disk surgery. Hill, G. M. et al., Clinical Orthopaedics and Related Researches 225: 229–233 (1987). However, surgical results are not always satisfying and importantly, this approach is far from optimal as removal of a lumbar intervertebral disk causes significant destabilization of the lower spine and predisposes adjacent intervertebral disks to degeneration in later years. Hill, G. M. et al., Clinical Orthopaedics and Related Researches 225: 229–233 (1987).
Recently, newer treatment modalities for chemonucleolysis have been developed that avoid some of the problems inherent in the use of proteases such as chymopapain and collagenases, and may avoid the need for surgical intervention. One such chemonucleolytic enzyme is chondroitinase ABC, a product of Proteus vulgaris. Chondroitinase ABC is an endo-N-acetyl-D-hexosaminidase that degrades mucopolysaccharides such as chondroitin sulfate, dermatan sulfate, chondroitin and hyaluronic acid. Takahashi et al., Spine 21:2405 (1996). Olmarker et al. note that chondroitinase ABC is much less injurious to spinal tissue than chymopapain. Spine 21:1952 (1996).
Osteogenic protein-1 (OP-1), also known as Bone Morphogenetic Protein-7 (BMP-7), is a member of the TGF-β superfamily that exerts potent effects on osteocyte and chondrocyte differentiation and metabolism. Asahina, I., et al., J. Cell Biol. 123(4): 921–33 (1993). The bone morphogenetic proteins were shown to induce new bone formation when injected subcutaneously in the rat. Cook, S. D. et al., Clin. Orthop. 324: 29–38 (1996). Recombinant human OP-1 (rhOP-1) has been shown to promote growth and differentiation of osteoblasts in vitro. Sampath, T. K., et al., J. Biol. Chem. 267: 20352–20360 (1992). It also causes differentiation of mesenchymal stem cells along chondrogenic and osteogenic pathways. Asahina, I., J. Cell. Biol. 123(4): 921–33 (1993). Recombinant human OP-1 also exerts specific effects on chondrocytes. Chen, et al. demonstrated that OP-1 promotes growth of chick sternal chondrocytes. Chen, P. et al., J. Cell. Sci. 108(Pt 1): 105–14 (1995). In this system, induction of synthesis of type X collagen was noted, suggesting that OP-1 exerted an effect on chondrocyte maturation as well. Bovine chondrocytes, in contrast, did not express type X collagen in response to OP-1 but did exhibit increased synthesis of PGs and type II collagen. Chen, P. et al., Biochem. Biophys. Res. Commun. 197(3): 1253–9 (1993). Growth factors are not used in existing chemonucleolysis treatment utilizing protease enzymes such as chymopapain and collagenase because these enzymes cleave or degrade the growth factors.
There is thus a need for a chemonucleolysis treatment that not only provides relief from the symptoms of intervertebral disk herniation, but also provides enhanced stability and repair of the AF and NP. Similarly, there is a need for a chemonucleolysis treatment that provides sufficient mechanical support to the spinal column, and that obviates the need for invasive surgical intervention. There is further a need to have a method of chemonucleolytic treatment that requires only one treatment modality for both dissolution of the intervertebral disk tissue while providing both stability and repair of that tissue. The treatment method of the present invention provides such a method.